1. Technical Field
The present invention relates to a digital amplifier, and more particularly to a circuit and a method of reducing pop-up noise in a digital amplifier.
2. Discussion of the Related Art
Audio amplifiers are typically classified as either class-A, class-B, class-AB, or class-D according to a driving circuit of the audio amplifier known as the output stage.
In the class-A output stage, bias voltages are applied to output transistors in a mute state, in which no signal is output, and a bias current flows through the output transistors in the mute state. Thus the class-A output stage has high thermal dissipation and low thermal efficiency. The class-B output stage is configured to prevent the bias current in the mute state. Significant crossover distortion, however, is caused in the class-B output stage when the output signal passes through a reference voltage, that is, when the output transistors are turned off. The class-AB output stage is configured to have a small bias current in the mute state to achieve the lower distortion of the class-A configuration and the higher efficiency of the class-B configuration. The crossover distortion, however, still exists and a high-capacity heat sink is required in the class-AB output stage.
The class-D output stage includes transistors operating as an ON/OFF switch. A turn-on resistance of the transistors is relatively small and thus the class-D output stage has a high efficiency. According to an IEC (International Electrotechnical Commission) standard, the class-D amplifier is defined as any amplifier, “in which the current in each active device supplying the load is switched from zero to a maximum value by a carrier signal, modulation of which conveys the useful signal.” Accordingly the class-D output stage includes all of the output stages, that is, the driving circuits are operated by a switching amplification regardless of whether an analog input or a digital input is received and regardless of the types of the internal signal processes used.
The audio amplifiers including such a class-D output stage or a class-D driving circuit are referred to as digital amplifiers or digital amps.
FIG. 1 is a diagram illustrating a conventional digital amplifier.
Referring to FIG. 1, the digital amplifier 100 includes a pulse width modulation (PWM) signal generator 10, a class-D driving circuit 20, a low-pass filter 30, and a coupling capacitor C2. Sound waves are generated through an output load RL coupled between an output node NA and a ground electrode. The output load RL is a load of a speaker 70 included in a headset, an earphone, etc.
The PWM signal generator 10 receives an audio signal of analog or digital type, and modulates the received audio signal to output a PWM signal. The PWM signal is a digital signal in which levels of the audio signal are transformed into time information of pulse widths.
The class-D driving circuit 20 receives and amplifies the PWM signal and outputs the amplified PWM signal. The class-D driving circuit 20 may be classified as either a half bridge or a full bridge in which two half bridges are combined. Under the same power supply voltage, the full bridge can provide a power two times greater than that of the half bridge. The power can be provided by a single power supply voltage or by two power supply voltages. Where the half bridge is used with the single power supply voltage, a DC voltage corresponding to a half of the power supply voltage is output and thus the coupling capacitor requires a high capacitance.
Power metal-oxide semiconductor field effect transistors (MOSFETs) of a complementary type, in which a P-channel transistor and an N-channel transistor are combined, are widely used as switching elements of the class-D driving circuit. The class-D driving circuit 20 may include a level shifter for increasing a reference level of the PWM signal, a deadtime circuit for preventing the two power MOSFETs from being simultaneously turned on, and a bootstrap circuit functioning as a floating power source.
The low-pass filter 30 averages a level of the output signal of the class-D driving circuit, and removes high bandwidth noise to output an analog signal to the output load RL of the speaker 70.
When the digital amplifier is powered on or off, a surge pulse may be generated due to instability of internal circuits and so on and the surge pulse is amplified by the driving circuit. The amplified surge pulse causes a noise, that is, an unpleasant sound through the speaker, and such noise is referred to as a pop-up noise.
In the digital amplifier 100 that is powered by a single power supply voltage, the pop-up noise can be reduced by coupling a relay between the low-pass filter 20 and the output load RL such that the relay is open until the digital amplifier is stabilized. The relay for reducing the pop-up noise, however, increases a size of the digital amplifier 100. Thus the relay is not particularly suitable for mobile devices.
A digital amplifier, in which a negative power supply voltage (−VDD) is generated by a charge pump or a DC-DC converter based on a power supply voltage (VDD), is disclosed in PCT patent publication No. 2006/031304. A driving circuit of the digital amplifier is operated by the two power supply voltages, that is, VDD and −VDD, and thus the coupling capacitor can be excluded. In such digital amplifiers without the coupling capacitor, the relay is ineffective for reducing the pop-up noise.